We have recently discovered a new human polyomavirus monoclonally-integrated into human Merkel cell carcinomas (MCC) that we call Merkel cell polyomavirus (MCPyV). MCC is a rare neuroectodermal cancer suspected to be caused by a viral infection because of its unusual epidemiology. It is the most aggressive skin cancer and only 50% of patients with advanced disease survive 9 months or longer. MCPyV has a 5.4 kbase genome closely related to murine and African green monkey lymphotropic polyomaviruses (MuPyV and LPyV, respectively). MCPyV is distantly related to SV40 and the four known human polyomaviruses. Human serosurveys show that 15-30% of populations from the US, Japan and Germany have cross-reactive antibodies to LPV which may actually represent reactivity to MCPyV infection. We find similar MCPyV infection rates using direct detection of MCPyV genome from peripheral blood cells. If confirmed, these findings suggest that over a billion persons have been exposed to MCPyV infections worldwide. Southern blotting shows that MCPyV is integrated into MCC genome at different sites in a somatic and monoclonal pattern. One cellular integration site has been defined as the receptor-type protein tyrosine phosphatase-gamma (PTPRG) intron 1. MCPyV also expresses a highly conserved T antigen in tumors. The N-terminus of MCPyV encodes transformation-associated DnaJ and LXCXE pocket protein-binding domains. All tumor- derived MCPyV T antigens, however, possess T antigen mutations that eliminate T antigen origin binding and/or plasmid replication functions. These functions are not needed to maintain integrated virus, suggesting that MCC arises in at least two steps: first, MCPyV integrates into the host genome; second, truncation mutations arise allowing expression of N-terminal transforming domains, but eliminating deleterious C- terminal domains. MCPyV may play a role in tumorigenesis through insertional mutagenesis, expression of T antigen or both. Our proposal seeks to understand these mechanisms for transformation and oncogenesis by 1) identifying additional cell integration sites, 2) analyzing T antigen transforming functions in rodent cells and in cell signaling assays, 3) performing cell-wide proteomic analysis following T antigen expression, 4) identifying novel cellular T antigen direct interactors and 5) generating transgenic mice with MCPyV T antigen expression targeted to Merkel mechanoreceptor cells. Through this systematic approach we anticipate we will learn how this new virus contributes to human carcinogenesis.